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Design >> Analog Design >> Problem in finding slew rate
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Message started by kanu on Jul 29^th, 2010, 6:04am

Title: Problem in finding slew rate
Post by kanu on Jul 29^th, 2010, 6:04am

Hi,

I tried finding slew rate of a 2-stage opamp.. but for some reasons the output waveform gives oscillations. I thought I would have switched the input terminals.. but that looks good. Bias circuit is fine too. It gives the constant voltage of 0.4 V to bias NMOS. What else could be a possible reason for such behavior?

I will appreciate any help.

Regards,
Kanu

Title: Re: Problem in finding slew rate
Post by kanu on Jul 29^th, 2010, 8:41am

Hi All,

One more question.. How does one find slew rate of an uncompensated opamp?

Regards,
Kanu

Title: Re: Problem in finding slew rate
Post by raja.cedt on Aug 2^nd, 2010, 10:20am

hi kanu,

method of finding slew rate for any class of an op with any method of compensation is same..because even if you don't have any compensation may be some parasitic cap will charge with ramp profile. Simulate and let me know i am sure you don't get into any problem.

Title: Re: Problem in finding slew rate
Post by thechopper on Aug 4^th, 2010, 4:48am

Are you testing the opamp in closed loop or open loop form?
You should try it in open loop, without any feedback network connected and simply switch the inputs with a large overdrive.
The ouput should switch from rail to rail with a certain slew rate.

Best
Tosei

Title: Re: Problem in finding slew rate
Post by kanu on Aug 5^th, 2010, 11:18am

Hi Tosei,

Thanks for your reply. I asked above question about uncompensated opamp, as I wanted to confirm if I measure transient response of opamp for pulse in open loop configuration.. will this be enough to determine its slew rate ? If so, then I don't need to worry. Otherwise first I will have to make my opamp stable for unity gain configuration :(

I think from your post, you do mean that transient response is enough. Do let me know if you think I have interpreted it wrong.

Thanks,
Kanu

Title: Re: Problem in finding slew rate
Post by thechopper on Aug 5^th, 2010, 5:54pm

Hi Kanu,

Yes, the transient response should be enough since slew rate is a large signal parameter and as such the best way to predict it is by transient analysis.

Best
Tosei